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Abstract:

An integrated digitally controlled linear-in-decibels attenuator circuit
in which one or more sets of selection switches establish a desired
attenuation by selectively connecting the input signal electrode to one
or more corresponding resistive ladder networks connected in series,
thereby providing a substantially more constant signal attenuation value
over a wider frequency bandwidth. With a single resistive ladder network,
attenuation control is achieved using a thermometer switching code. With
multiple resistive ladder networks, coarse and fine attenuation control
can be achieved using thermometer and bubble switching codes,
respectively.

Claims:

1.-20. (canceled)

21. An apparatus including an integrated digitally controlled
linear-in-decibels attenuator circuit, comprising:first switched
resistance circuitry responsive to a first plurality of digital control
signals by attenuating an input signal to provide a first attenuated
signal having a magnitude substantially in accordance with one of a first
plurality of attenuation values mutually separated by a first attenuation
step substantially in accordance with a thermometer code; andsecond
switched resistance circuitry coupled to said first switched resistance
circuitry and responsive to a second plurality of digital control signals
by attenuating said first attenuated signal to provide a second
attenuated signal having a magnitude substantially in accordance with one
of a second plurality of attenuation values mutually separated by a
second attenuation step substantially in accordance with a bubble code,
wherein said second attenuation step is smaller than said first
attenuation step;wherein a ratio of said second attenuated signal and
said input signal is one of a plurality of ratio values mutually
separated by linear-in-decibels steps.

22. The apparatus of claim 21, wherein:said first switched resistance
circuitry is responsive to said first plurality of digital control
signals during a first time interval such that said first attenuated
signal magnitude transitions through a first plurality of signal
magnitudes; andsaid second switched resistance circuitry is responsive to
said second plurality of digital control signals during a second time
interval such that said second attenuated signal magnitude transitions
through a second plurality of signal magnitudes.

23. The apparatus of claim 22, wherein:said first plurality of signal
magnitudes are mutually related by said first attenuation step; and said
second plurality of signal magnitudes are mutually related by said second
attenuation step.

25. The apparatus of claim 21, wherein:said first switched resistance
circuitry includes M stages of switched resistances;said second switched
resistance circuitry includes N stages of switched resistances; andsaid
first and second switched resistance circuitries together provide M*N
attenuation values.

26. The apparatus of claim 21, wherein:said first switched resistance
circuitry includesa first plurality of series resistances each of which
having a resistance Rs1 related to a reference resistance Rr and a first
scalar A substantially in accordance with Rs1=Rr/A-Rr, anda first
plurality of shunt resistances each of which having a resistance Rp1
related to said reference resistance Rr and said first scalar A
substantially in accordance with Rp1=Rr/(1=A); andsaid second switched
resistance circuitry includesa second plurality of series resistances
each of which having a resistance Rs2 substantially equal to said
reference resistance Rr, anda second plurality of shunt resistances each
of which having a resistance Rp2 related to said reference resistance Rr
and a second scalar K substantially in accordance with
Rp2=(ReK/(1-K))*((Rr*K/(1-K))+Rr)/Rr.

27. A method for attenuating a signal in a linear-in-decibels manner in
accordance with a plurality of digital control signals,
comprising:attenuating an input signal in accordance with a first
plurality of digital control signals to provide a first attenuated signal
having a magnitude substantially in accordance with one of a first
plurality of attenuation values mutually separated by a first attenuation
step substantially in accordance with a thermometer code; andattenuating
said first attenuated signal in accordance with a second plurality of
digital control signals to provide a second attenuated signal having a
magnitude substantially in accordance with one of a second plurality of
attenuation values mutually separated by a second attenuation step
substantially in accordance with a bubble code, wherein said second
attenuation step is smaller than said first attenuation step;wherein a
ratio of said second attenuated signal and said input signal is one of a
plurality of ratio values mutually separated by linear-in-decibels steps.

28. The method of claim 27, wherein:said attenuating an input signal
comprises attenuating said input signal in accordance with said first
plurality of digital control signals during a first time interval such
that said first attenuated signal magnitude transitions through a first
plurality of signal magnitudes; andsaid attenuating said first attenuated
signal comprises attenuating said first attenuated signal in accordance
with said second plurality of digital control signals during a second
time interval such that said second attenuated signal magnitude
transitions through a second plurality of signal magnitudes.

29. The method of claim 28, wherein:said first plurality of signal
magnitudes are mutually related by said first attenuation step; andsaid
second plurality of signal magnitudes are mutually related by said second
attenuation step.

32. The method of claim 27, wherein:said attenuating an input signal
comprises switching said input signal among M stages of switched
resistances includinga first plurality of series resistances each of
which having a resistance Rs1 related to a reference resistance Rr and a
first scalar A substantially in accordance with Rs1=Rr/A-Rr, anda first
plurality of shunt resistances each of which having a resistance Rp1
related to said reference resistance Rr and said first scalar A
substantially in accordance with Rp1=Rr/(1-A); andsaid attenuating said
first attenuated signal comprises switching said first attenuated signal
among N stages of switched resistances includinga second plurality of
series resistances each of which having a resistance Rs2 substantially
equal to said reference resistance Rr, anda second plurality of shunt
resistances each of which having a resistance Rp2 related to said
reference resistance Rr and a second scalar K substantially in accordance
with Rp2=(Rr*K/(1-K))*((Rr*KJ(1-K))+Rr)/Rr.

[0005]Digitally controlled attenuator circuits are well-known in the art.
Such attenuator circuits are generally used in controlled impedance
environments, and allow the attenuation to be controlled in units or
fractions of decibels (dB). One particular type of such attenuator is
referred to as a linear-in-dB attenuator, in which a thermometer code
type of switching, or control, signal causes the attenuation to vary in
single dB steps.

[0006]Referring to FIG. 1, a conventional digitally controlled
linear-in-dB attenuator includes a resistive ladder circuit with series
resistances Rs2-Rs7 and shunt resistances Rp1-Rp7, interconnected
substantially as shown, to which the input voltage signal Vin is applied.
The voltages at nodes N1-N7 are applied to the throw electrodes of the
single-pole, single-throw switch circuits S1-S7. The pole electrodes of
these switches S1-S7 are mutually connected to provide the output signal
Vout. The switches S1-S7 are controlled with a thermometer code control
signal to selectively close the individual switches, depending upon the
desired attenuation. (As one example embodiment, the series resistances
Rs2-Rs7 would have nominal resistance values of 109 ohms, while the shunt
resistances Rp1-Rp7 would have nominal resistances of 8170 ohms.)

[0007]Referring to FIG. 1A, a problem with such conventional attenuator
circuits is the limited bandwidth caused by the circuit topology. As seen
in FIG. 1A, at or near a certain frequency Fc, the attenuation is no
longer constant and begins to increase. This is due to the switch
circuits S1-S7, which are typically implemented using metal oxide
semiconductor field effect transistor (MOSFET) switches with low turn-on
resistances. As is well-known in the art, such devices typically have
relatively high parasitic capacitances at their drain and source
electrodes. It is this parasitic capacitance that causes the bandwidth to
be limited, thereby causing the attenuation characteristics to no longer
be constant above a certain frequency Fc. Further, also as shown in FIG.
1A, the bandwidth decreases as the attenuation increases. This is caused
by the increased capacitance due to more of the switches S1-S7 being in
their off states.

SUMMARY OF THE INVENTION

[0008]An integrated digitally controlled linear-in-decibels attenuator
circuit in which one or more sets of selection switches establish a
desired attenuation by selectively connecting the input signal electrode
to one or more corresponding resistive ladder networks connected in
series, thereby providing a substantially more constant signal
attenuation value over a wider frequency bandwidth. With a single
resistive ladder network, attenuation control is achieved using a
thermometer switching code. With multiple resistive ladder networks,
coarse and fine attenuation control can be achieved using thermometer and
bubble switching codes, respectively.

[0010]a plurality of attenuation control electrodes to convey a plurality
of digital control signals corresponding to a signal attenuation value in
accordance with a thermometer code;

[0011]an input signal electrode to convey an input signal having a
magnitude; an output signal electrode to convey an output signal
corresponding to the input signal and having a magnitude which is less
than the input signal magnitude in relation to the signal attenuation
value; and

[0012]a resistive network coupled between the input and output signal
electrodes and responsive to the plurality of digital control signals by
attenuating the input signal to provide the output signal.

[0014]a first plurality of attenuation control electrodes to convey a
first plurality of digital control signals corresponding to a first
signal attenuation value in accordance with a thermometer code;

[0015]a second plurality of attenuation control electrodes to convey a
second plurality of digital control signals corresponding to a second
signal attenuation value in accordance with a bubble code;

[0016]an input signal electrode to convey an input signal having a
magnitude;

[0017]an intermediate signal electrode to convey an intermediate signal
corresponding to the input signal and having a magnitude which is less
than the input signal magnitude in relation to the first signal
attenuation value;

[0018]an output signal electrode to convey an output signal corresponding
to the intermediate signal and having a magnitude which is less than the
intermediate signal magnitude in relation to the second signal
attenuation value;

[0019]a first resistive ladder network coupled between the input and
intermediate signal electrodes and responsive to the first plurality of
digital control signals by attenuating the input signal to provide the
intermediate signal; and

[0020]a second resistive ladder network coupled between the intermediate
and output signal electrodes and responsive to the second plurality of
digital control signals by attenuating the intermediate signal to provide
the output signal.

[0022]FIG. 1A is a graph of attenuation versus frequency for the circuit
of FIG. 1.

[0023]FIG. 2 is a schematic diagram of a digitally controlled linear-in-dB
attenuator circuit in accordance with one embodiment of the presently
claimed invention.

[0024]FIG. 2A is a graph of attenuation versus frequency for the circuit
of FIG. 2.

[0025]FIG. 3 is a schematic diagram of one example of an implementation of
a switch circuit for the attenuator circuit of FIG. 2.

[0026]FIG. 4 is a block diagram of a digitally controlled linear-in-dB
attenuator circuit in accordance with another embodiment of the presently
claimed invention.

[0027]FIG. 5 is a table of thermometer and bubble codes for attenuator
control signals in accordance with one embodiment of the presently
claimed invention.

[0028]FIG. 6 is a graph of attenuation levels versus time for the
attenuator circuit of FIG. 4 with the attenuator control signals of FIG.
5.

DETAILED DESCRIPTION

[0029]The following detailed description is of example embodiments of the
presently claimed invention with references to the accompanying drawings.
Such description is intended to be illustrative and not limiting with
respect to the scope of the present invention. Such embodiments are
described in sufficient detail to enable one of ordinary skill in the art
to practice the subject invention, and it will be understood that other
embodiments may be practiced with some variations without departing from
the spirit or scope of the subject invention.

[0030]Throughout the present disclosure, absent a clear indication to the
contrary from the context, it will be understood that individual circuit
elements as described may be singular or plural in number. For example,
the terms "circuit" and "circuitry" may include either a single component
or a plurality of components, which are either active and/or passive and
are connected or otherwise coupled together (e.g., as one or more
integrated circuit chips) to provide the described function.
Additionally, the term "signal" may refer to one or more currents, one or
more voltages, or a data signal. Within the drawings, like or related
elements will have like or related alpha, numeric or alphanumeric
designators. Further, while the present invention has been discussed in
the context of implementations using discrete electronic circuitry
(preferably in the form of one or more integrated circuit chips), the
functions of any part of such circuitry may alternatively be implemented
using one or more appropriately programmed processors, depending upon the
signal frequencies or data rates to be processed.

[0031]Referring to FIG. 2, an integrated digitally controlled linear-in-dB
attenuator circuit in accordance with one embodiment of the presently
claimed invention includes a resistive ladder circuit, with series
resistances Rs2-Rs7 and shunt resistances Rp1-Rp7, and single-pole,
double- throw switch circuits S1-S6, all interconnected substantially as
shown. (It will be understood by one of ordinary skill in the art that
fewer or more series and shunt resistances and switch circuits can be
used in accordance with the number of dB steps of attenuation desired.)
The input signal Vin is applied to the series resistances Rs2-Rs7 via
resistance Rp7, and to resistances Rp1-Rp6 via the switch circuits S1-S6.
Accordingly, the output signal Vout is provided at the output of the
resistive ladder circuit (e.g., as opposed to the mutually connected pole
electrodes of the switch circuits S1-S6). In conformance with Thevenin's
Theorem, this circuit topology advantageously maintains a sufficient
output impedance at the output node No since the pole electrodes of the
switch circuits S1-S6 are isolated from the output node No by the shunt
Rp1-Rp6 and series Rs2-Rs6 resistances, and the throw electrodes are
connected either to the low impedance input node Ni or to low impedance
circuit ground GND, depending upon the desired signal attenuation.

[0032]Referring to FIG. 2A, as a result of this circuit topology, the
signal attenuation remains more constant over a wider frequency bandwidth
due to the isolation of the parasitic capacitances of the switch circuits
S1-S6 from the output node No.

[0033]Referring to FIG. 3, an example embodiment of a switch circuit,
e.g., the first switch circuit S1, includes pairs of N-type and P-type
MOSFETs interconnected as transmission gates. For example, complementary
pairs N1, P1 and N2, P2 of MOS transistors are interconnected with
mutually coupled drain and source electrodes as shown. The incoming
control signal drives the gate electrodes of transistors N1 and P2, while
the inverted control signal (inverted by an inverter circuit INV) drives
the gate electrodes of transistors P1 and N2. Hence, when the control
signal is asserted high, the N1-P1 transistor pair is turned on while the
N2-P2 transistor pair is turned off. Conversely, when the control signal
is de-asserted low, transistor pair N2-P2 is turned on while transistor
pair N1-P1 is turned off. Alternatively, instead of transmission gates,
single transistors can be used as pass transistors. For example,
transistors N1 and P2 can be used with transistors P1 and N2 omitted.

[0034]Referring to FIG. 4, a digitally controlled linear-in-dB attenuator
circuit 400 in accordance with another embodiment of the presently
claimed invention includes at least two stages 200, 100 connected in
series, with the first stage 200 being a circuit in conformance with FIG.
2, and the second stage 100 being a circuit in conformance with FIG. 1
(with the output node No of FIG. 2 connected to the input node N7 of FIG.
1). Accordingly, with the two stages 200, 100 implemented as the example
circuits of FIGS. 2 and 1, the first stage 200 will have M=6 stages of
resistive attenuators (as well as M=6 switches) for M steps of coarse
adjustment, and the second stage 100 will have N=7 stages of resistive
attenuators (as well as N=7 switches) for N steps of fine adjustment.
This results in having M*N=42 possible adjustments while needing only
M+N=13 stages of resistive attenuators (with M+N=13 switches), which is
significantly less than M*N=42 stages of resistive attenuators (as well
as M*N=42 switches) as required in a conventional linear-in-dB attenuator
circuit.

[0035]Based upon a reference resistance value Rref, preferred relative
values of the resistances in the first stage 200 (Rs2-Rs7 and Rp1-Rp7)
and second stage 100 (Rs2-Rs7 and Rp1-Rp7) are as follows (where a<1
and k<1):

[0038]For example, with a reference resistance value of Rref=500, and
a=0.9441 and k=0.7079, coarse and fine steps of 3 dB and 0.5 dB,
respectively, can be realized.

[0039]While the minimum attenuation of such an attenuator circuit 400 is
equal to one coarse attenuation step and not zero (0 dB), due to
resistors Rp7 and Rs2-Rs7 in the first stage 200 (FIG. 2), it will be
readily appreciated by one of ordinary skill in the art that such minimum
signal loss can be compensated by the gain of an output buffer amplifier
(not shown) following the second attenuator stage 100.

[0040]Referring to FIG. 5, in accordance with one embodiment of the
presently claimed invention, the attenuator control signals, i.e., the
switch control signals CONTROL (FIGS. 1 and 2), for a digitally
controlled linear-in-dB attenuator circuit 400 in accordance with FIG. 4
would be as shown for a first stage 200 having M=5 stages of resistive
attenuators and M=5 switches for M steps of coarse adjustment (i.e.,
switch S6 and resistances Rp6, Rp7 and Rs7 are not used, and the input
signal Vin is applied to resistance Rs6), and a second stage 100 having
N=7 stages of resistive attenuators and N=7 switches for N steps of fine
adjustment. Also in accordance with the presently claimed invention, the
first stage 200 provides coarse attenuation control in accordance with
thermometer code, while the second stage 100 provides fine attenuation
control in accordance with bubble code.

[0041]In the case of the first stage 200 providing coarse attenuation
control, an advantage to using thermometer code for such a R-2R resistive
ladder network is the ability to provide linear-in-dB attenuation. This
is in contrast to the use of binary code which would provide
linear-in-voltage control.

[0042]Referring to FIG. 6, attenuation levels versus time are shown for
the attenuator circuit of FIG. 4 using the thermometer and bubble codes
of FIG. 5 for the attenuator control signals.

[0043]Various other modifications and alternations in the structure and
method of operation of this invention will be apparent to those skilled
in the art without departing from the scope and the spirit of the
invention. Although the invention has been described in connection with
specific preferred embodiments, it should be understood that the
invention as claimed should not be unduly limited to such specific
embodiments. It is intended that the following claims define the scope of
the present invention and that structures and methods within the scope of
these claims and their equivalents be covered thereby.